Known as a solid hitter and a good fielder, first baseman Pete O'Brien played a dozen seasons in the majors, mostly with the Texas Rangers.

Selected in the 15th round of the 1979 amateur draft, O'Brien was originally signed by scout Lee Anthony and made his pro debut that summer with the GCL Rangers. After hitting .310 with 25 home runs for the Denver Bears in 1982, he earned a September call-up to the big leagues, batting .239 in 20 games. He was a regular at first for Texas the next year, 1983, hitting .237 as a rookie. In 1984, he raised his batting average by 50 points and clubbed 18 homers. He had perhaps his finest year in 1986, hitting .290 with 23 home runs.

Following the 1988 season, O'Brien was traded along with two others to the Cleveland Indians for Julio Franco. After one summer in Cleveland in 1989, he became a free agent and signed a four-year deal with the Seattle Mariners. He struggled with injuries in his first season with the M's in 1990, hitting just .224 with 5 homers in 108 games, but bounced back in 1991, hitting 17 home runs and leading American League first baseman with a .997 fielding percentage. However, with the emergence of Tino Martinez, O'Brien's role was reduced, and he was released in 1993.

In 2014, he was back with the Rangers as a special instructor during spring training.